This invention relates to a method of processing a semiconductor device and in particular to a method for processing a semiconductor device to provide an integrated circuit that includes a functional resistor. Although some aspects of this invention may be applied to semiconductor devices other than charge-coupled devices (CCDs), certain aspects of the invention are particularly useful when applied to a CCD and therefore the invention will be described in the context of a CCD. Further, it will be understood by those skilled in the art that there are many different types of CCDs and those skilled in the art will understand that the invention described herein, even as applied to a CCD, is not limited in application to a specific type of CCD. Therefore, it should not be inferred from the fact that the invention is described with reference to a back-side illuminated, n-channel three-phase device that the invention is limited in application to a CCD, or that the invention, even as applied to a CCD, is limited to this specific type of CCD.
A charge-coupled device (CCD) may be made by processing a silicon die of p conductivity using conventional MOS technology to form a buried channel of n conductivity in an active region beneath the front or circuit side of the die (the side through which the die is processed). The channel is resolved into a linear array of like elementary zones by a clocking electrode structure which is composed of gate electrodes and overlies the circuit side of the die, and by application of selected potentials to the gate electrodes, a charge packet present in a given elementary zone of the channel may be advanced through the linear array of elementary zones, in the manner of a shift register, and discharged from the channel. In a multi-phase CCD, the gate electrodes are organized as multiple sets and different phases of a multi-phase clock signal are applied to the respective sets of gate electrodes.
Charge may be generated in the channel photoelectrically. Thus, if electromagnetic radiation enters the buried channel, it may cause generation of conduction electrons, and these conduction electrons may be confined in one of the elementary zones to form a charge packet.
A CCD may be used to generate an electrical signal representative of the distribution of light intensity over the active region of the CCD. In such an imaging CCD, there may be multiple imaging channels extending parallel to one another and each connected at one end to a common readout channel which extends perpendicular to the imaging channels. Charge packets are generated in the elementary zones or pixels of the imaging channels during an integration interval. Subsequently, during a readout interval, the charge packets are transferred from the imaging channels into the readout channel and the charge packets are transferred serially through the readout channel and deposited in an N+ floating diffusion. The size of a charge packet is measured by using a charge-sensing amplifier to sense the potential of the floating diffusion and the floating diffusion is then reset by a reset FET, with respect to which the floating diffusion acts as source and a reset diffusion acts as drain.
Referring to FIG. 1, the floating diffusion 2 is connected to the charge-sensing amplifier, which is typically implemented by a MOSFET 4 operating in the source follower configuration and developing an output signal across a load resistor 6. The reset FET 16 has a gate RG for selectively connecting the floating diffusion 2 to the reset diffusion 18.
In general, it is desirable that the source follower MOSFET 4 should generate a low noise output signal. To maintain a low noise output, the conversion gain of the floating diffusion needs to be high. This is achieved by minimizing the capacitance of the floating diffusion and all its associated parasitics. Since the gate capacitance of the MOSFET contributes directly to the total parasitic capacitance, in order to generate a low noise output signal, the source follower MOSFET should be small. On the other hand, the bandwidth of the source follower is limited by the capacitance that the MOSFET can drive. In a typical implementation of the arrangement shown in FIG. 1, the load resistor 6 is off-chip and therefore there can be a rather large parasitic capacitance associated with the load resistor. Accordingly, although the output structure shown in FIG. 1 operates well for devices designed to function at a relatively low rate (less than 500 k pixels/sec.), it is not optimal for higher speed applications.
In order to operate at higher data rates, it is necessary to provide an output amplifier with a higher bandwidth. This can be accomplished by constructing a multistage source follower amplifier 8, as shown in FIG. 2. In order to maintain noise performance and high gain, it is important that the first stage MOSFET 10 be made small. Consequently, in order to operate at a high data rate, it is usual to employ a multistage source follower amplifier with the first stage being small and successive stages increasingly larger. For example, in one CCD that has been manufactured employing a two-stage amplifier, the first stage MOSFET 10 has l/w (length/width)=3/17 and the second stage MOSFET 12 has l/w=3/200.
Each source follower MOSFET in the multistage source follower amplifier requires its own load resistor. The load resistor 14 of the final MOSFET 12 may be off-chip and the loads for the first stage and any intermediate stages are typically provided by large MOSFETs (l/w=20/20) acting as constant current sources. This is a desirable configuration with respect to gain.
The current source MOSFETs serving as load transistors are fabricated on-chip, reducing the parasitic capacitance presented to the first stage and achieving a high bandwidth. Such loads are satisfactory for many applications where noise is not a severe limitation, but for low noise applications, such on-chip current sources are not acceptable because they are not only noisy but also tend to glow. The impact of glowing can be mitigated by placing the load transistor off-chip, but this will increase the parasitic capacitance and therefore reduce the bandwidth of the circuit. With proper design, it is possible to reduce the noise generated by the current source, but at the cost of speed.
An alternative to using load transistors as loads for the source follower MOSFETs is to employ on-chip resistors. For example, polysilicon resistors have been used in CMOS circuits. However, thin film resistors have better noise characteristics than polysilicon resistors.
A known method of fabricating a multiphase imaging CCD will now be described with reference to FIGS. 3-7.
FIG. 3 shows a silicon single crystal die 22 that has been processed in conventional fashion to form an active region 24 which extends partly into the die from the front side 26 thereof. The active region contains the imaging channels and the readout channel, but the channels are not shown in FIG. 3. The active region 24 is surrounded by a thick layer of field oxide 28. There are several apertures 30 (only two of which are shown in FIG. 3) in the field oxide near the periphery of the die. There is a thin layer of gate oxide (not shown in FIG. 3) over the die in the active region 24 and in the apertures 30.
Referring to FIG. 4, the clocking electrode structure includes three sets of polysilicon conductor strips 32.sub.1, 32.sub.2 and 32.sub.3, corresponding respectively to the three phases of the clock signal used to operate the CCD. The conductor strips 32.sub.1, 32.sub.2 and 32.sub.3 include respective gate electrodes 34.sub.1, 34.sub.2 and 34.sub.3 (FIG. 6) which extend over the thin oxide 36, crossing the channels that are influenced by the gate electrodes. Each conductor strip includes a gate extension 38 (FIG. 5) which extends some distance over the field oxide. The conductor strips 32 of the three sets are formed sequentially, by depositing and patterning three successive layers of polysilicon, and the three deposits of polysilicon are referred to as the first, second and third levels, in accordance with the order in which they are deposited. The first level polysilicon includes a bus 40 which connects the conductor strips 32.sub.1.
Referring to FIG. 5, discrete islands 42 of polysilicon extend into the apertures 30 in the field oxide.
When the polysilicon is initially deposited and patterned, it is non-conductive. Conductivity is imparted to the polysilicon conductor strips 32, the polysilicon bus 40 and the polysilicon islands 42 by doping with a donor dopant, such as phosphorus.
After the three levels of polysilicon have been deposited, patterned and doped, a layer 50 of reflow glass is deposited over the upper surface of the device and is patterned to expose various parts of the polysilicon conductor runs, including portions of the polysilicon bus 40, terminal portions of the second and third level polysilicon conductor strips 32.sub.2 and 32.sub.3 and the polysilicon islands 42. A blanket layer of interconnect metal is deposited over the reflow glass and is patterned to provide first, second and third buses 52.sub.1, 52.sub.2 and 52.sub.3. The first metal bus 52.sub.1 overlies the polysilicon bus 40 and is connected thereto by vias 54.sub.1, extending through apertures in the reflow glass. The second metal bus 52.sub.2 extends parallel to the first metal bus and is connected to the individual conductor strips 32.sub.2 of second level polysilicon by vias 54.sub.2 extending through apertures in the reflow glass. Similarly, the third metal bus 52.sub.3 extends parallel to the first metal bus and is connected to the conductor strips 32.sub.3 of third level polysilicon by vias 54.sub.3. Each of the metal buses 52 has a connection branch 56 extending outward to one of the polysilicon islands 42. Patterning of the interconnect metal may provide other interconnections than those mentioned above, such as the connection between the floating diffusion 2 (FIGS. 1 and 2) and the gate of the MOSFET 4 or 10.
An interface layer 58 is deposited over the front side of this structure and a support 60 is attached to the interface layer. The interface layer 58 accommodates differential thermal expansion and provides an adhesion layer between the silicon die 22 and the support 60. The silicon die is then thinned from its back side to a thickness in the range from about 10 to 20 .mu.m. After thinning, the active region of the thinned die is masked and portions of the substrate outward of the active region are completely removed, leaving a silicon plateau 22' containing the active region surrounded by a plain of field oxide 28. In FIG. 5, the topography of the plateau and plain are inverted since, by convention, the circuit side of the structure is shown upward. The gate oxide is removed from the apertures 30 in the field oxide and aluminum is deposited over the plain of field oxide and is patterned to provide bonding pads including portions 62 which extend into the apertures 30 and make contact with the polysilicon islands 42. The aluminum bonding pads 62 are connected by wire bonding to external circuitry for driving the gate electrodes. See U.S. Pat. Nos. 4,923,825 and 6,072,204.
FIG. 7A is a partial top plan view of the die prior to deposit of the interconnect metal and shows five of the polysilicon islands 42. Further, FIG. 7A shows apertures 64 in the reflow glass 50 through which source and drain regions of the reset FET 16, the first stage MOSFET 10 and the second stage MOSFET 12 are exposed. The FETs 10, 12 and 16 have polysilicon gates 66, 68 and 70 respectively, which also are partially exposed through apertures in the reflow glass.
FIG. 7B shows the same part of the die after the interconnect metal has been deposited and patterned. Conductor runs of interconnect metal connect the gate and drain of the reset FET 16 to respective polysilicon islands 42.sub.1, and 42.sub.2 and connect the source of the reset FET to the gate 66 of the MOSFET 10. Similarly, interconnect metal connects the drain of the MOSFET 10 to an island 42.sub.3 and connects the source of the MOSFET 10 to the gate of the MOSFET 12. Interconnect metal (not shown) connects the source of the MOSFET 10 to the load for that MOSFET. Further, interconnect metal connects the drain and source of the MOSFET 12 to the polysilicon island 42.sub.4 and 42.sub.5. When the device is completed, the polysilicon islands 42.sub.1 and 42.sub.2 are connected through the respective bond pads 62 and bond wires to the reset gate control signal and the reset drain reference potential level. The polysilicon islands 42.sub.3 and 42.sub.4 are connected to the VDD reference potential level and the island 42.sub.5 is connected to an output terminal of the device.
In one practical implementation of the process described with reference to FIGS. 3-7, the support 60 is made of borosilicate glass. The process by which the borosilicate glass support is formed involves high temperature steps and consequently the interconnect metal must be a refractory metal rather than the aluminum that is frequently used to provide conductive traces in semiconductor device fabrication. Other techniques for supporting the silicon die during thinning, without requiring high temperature process steps, have also been proposed. In the event that the process steps are below about 450.degree. C., the interconnect metal that is used to provide the buses and other connections may be aluminum.
Aluminum is reactive with silicon. Therefore, when aluminum is used as interconnect metal to provide a connection to a silicon contact, it is desirable that a thin layer of barrier conductor be provided between the interconnect metal and the silicon contact to prevent the reaction that would otherwise take place at the junction between the aluminum interconnect metal and the silicon single crystal, since this reaction may damage the silicon crystal and impair the performance of the device. The barrier conductor may be, for example, doped silicon, titanium nitride or titanium/tungsten. Some of the materials that are suitable for use as the barrier conductor have a high resistivity compared with aluminum. Conventionally, the barrier conductor is patterned at the same time as the aluminum so that the barrier conductor and the aluminum are essentially coextensive.
Copper may be used as an interconnect metal in the event that the interconnect metal is not required to withstand high temperatures. It is necessary to provide a conductive barrier layer between copper interconnect metal and the silicon crystal.
All known electrically-conductive materials have finite resistivity at and above normal room temperature (about 18.degree. C.), but for some metal structures the electrical resistance is so low that it is considered negligible for most circuit design purposes. Most electrical circuits include at least one circuit element that provides a specific and defined function as a resistor in the electrical circuit. In current integrated circuit design practice, a circuit element would have to have an electrical resistance of at least about 10 ohm in order to be considered to be a functional resistor.